The use of airships as high altitude platforms for a number of applications has long been contemplated. It has been suggested that airships could provide suitable platforms for communication and surveillance purposes or for environmental applications (e.g. monitoring of pollution or the ozone layer). Traditional airships, whether non-rigid airships or those having a rigid internal frame or skeleton, can be more suitable for use at low altitudes than at high altitudes.
In one common form of an airship, a gondola which may house crew, equipment, etc., is suspended under an elongate generally ellipsoidal or cigar shaped envelope filled with lighter-than-air or lifting gas, such as, for example, hydrogen or helium. Such traditional cigar-shaped airships are typically not used at altitudes above approximately 5,000 feet or approximately 1.5 kilometers (Km) above mean sea level. Such airships are typically used for advertising, relaying broadcasts of events, monitoring, security guarding, transporting, sightseeing, etc. at low altitudes (approximately 1 kilometer or less above mean sea level) where there is a relatively small change in atmospheric pressure. For airships used at such low altitudes, the flight altitude control is relatively easy because the flight altitude does not have to be changed over a wide range. Specifically, for such an airship, the volume of the envelope containing the lighter-than-air or lifting gas is determined so that it can withstand flight at the maximum altitude. A traditional cigar-shaped airship ascends by gaining speed from its propulsion apparatus, such as, for example, engines or propellers and then using the elevators, located at the rear of the. horizontal fins, to give the airship a positive or nose-up pitch. If the airship is equipped with vectoring engines/propellers it can also ascend by the vectored thrust.
Unlike rigid airships which have an internal framework, non-rigid airships maintain their shape solely through pressure exerted on the interior surface of an envelope by the fluids (e.g. lifting gas and/or air) contained within the envelope. This pressure is exerted through a combination of lifting gas contained within the envelope and air contained within interior envelopes, or ballonets, mounted within the envelope. Conventionally, cigar-shaped airships have one or more ballonets of variable volume mounted within the envelope. Ballonets are structures contained within the envelope of the airship and are adapted to receive air from the exterior of the airship. They also act to maintain the pressure exerted on the interior surface of the envelopes so as to maintain the shape of the airship. The volume of air contained within each ballonet can be adjusted by inflating or deflating the ballonet. In this way, the pressure exerted on the interior surface of the envelope can be controlled, as changes to the volume of the ballonets compensates for changes in the volume of the lighter-than-air or lifting gas contained within the envelope that occur upon altitude changes. Furthermore, the pressure exerted on the interior surface of the envelope can be adjusted in accordance with the ambient pressure about the exterior surface of the airship so as to usually create and/or maintain a constant pressure differential between the internal pressure within the envelope and the external ambient pressure. This serves to maintain appropriate fluid (e.g. gas) pressure on the envelope and accordingly, the shape of the airship, preventing deformation or structural failure. This pressure differential is typically regulated or maintained automatically through blowers that are designed to provide a specific pressure and through valves that open when the pressure exceeds a predetermined limit.
In order for an airship to ascend to altitudes of between about 60,000 ft. to 70,000 ft., typically referred to as the “stratosphere” (e.g., approximately 18 to 21 Km), where the atmosphere density is approximately 1/9 to approximately 1/19 of that in the vicinity of the mean sea level, it is indispensable to provide an airship with a mechanism capable of adjusting to the varying volume (e.g. approximately 9 to approximately 19 fold) of the buoyant or lifting gas (e.g. helium, hydrogen, etc.). The interior or internal volume of the airship must therefore accommodate the expansion of the lifting gas that will occur as the airship gains altitude. For example, the ballonet operation between mean sea level (where ambient pressure is about 1013 mBar (MB)) and 5,000 ft (where ambient pressure is about 843 mBar (MB)) may involve ballonet(s) of approximately 20% of the interior or internal volume of the airship. In other words, when the ballonet(s) are close to being fully inflated, near mean sea level, they occupy approximately 20% of the internal or interior volume of the envelope of the airship. As the airship ascends from sea level, the lifting gas expands and the ballonet contracts. When the ballonet is empty the airship is at pressure altitude and can not ascend any higher without risking the rupture of the airship's envelope as a result of the increasing pressure of the lifting gas that now has nowhere to expand.
For a service ceiling of about 65,000 ft (where the ambient pressure is about 56 MB), the volume of the lifting gas used at lift-off from mean sea level may be as little as approximately 1/14 of the volume of the lifting gas at 65,000 ft. Therefore, at low altitude, the ballonets will tend to occupy a greater portion of the internal or interior volume of the airship and the lifting gas will occupy only a small portion of the internal or interior volume of the airship. This may present significant control challenges at low altitude, particularly for cigar shaped airships. This relatively small volume of lifting gas, which may occupy only about 6% of the volume of the airship at sea level, could be difficult to confine with traditional ballonets and, as a result, can shift within the envelope, affecting the airship's pitch (e.g. trim), yaw or rotation (collectively referred to as “attitude”) and causing destabilization of the airship. As a result, traditionally designed cigar-shaped airships have been considered impractical for use at high altitude.
Attempts have been made to limit the destabilization that occurs in cigar-shaped airships, particularly airships designed for high altitude flight. Differential inflation of the ballonets can be used to adjust airship trim and thus maintain stability. In some cases, rather than mounting ballonets at the centre of the airship, ballonets are positioned in the front and rear sections of the hull. The supply or discharge of air to or from specific ballonets attempts to compensate for attitude instability of the airship as the lifting gas expands or shifts within the envelope.
Further attempts have been made to improve the efficiency of adjusting airship pitch (e.g. trim) and thus improve stability by positioning multiple ballonets along the entire length of the hull of the airship, to provide rapid attitude adjustment. For example, U.S. Pat. No. 6,698,686 to Ogawa et al. provides an airship in which the hull is divided vertically by bulkheads into a plurality of compartments that hold lifting gas in their upper sections and air in their lower sections. The bulkheads are formed of a meshed sheet in the upper sections and are provided with a plurality of vents to allow the lifting gas to move between respective upper compartments. When the lifting gas expands or moves between upper compartments to cause destabilization of the airship, air supply-and-discharge devices are used to alter the quantity of air in respective lower compartments to change the mass balance and thus stabilize the airship.
In a stratospheric airship described in U.S. Pat. No. 6,427,943 to Yokomaku et al., a diaphragm divides the interior of a high altitude airship into a buoyant gas compartment and an air compartment. The diaphragm is kept taut across a horizontal axis of the airship by a suspension cord connected to the upper and lower surfaces of the hull of the airship. This allows for smooth change in shape of the diaphragm as the buoyant gas expands or moves about, thus reducing the movement of the lifting gas that causes destabilization.
U.S. Pat. No. 5,294,076 as well as U.S. patent application Ser. Nos. 10/178,345 and 10/718,634, the content of which are incorporated herein by reference, provide examples of spherical airships directed to high altitude uses.
It would be advantageous to provide a generally elongated cigar shaped airship in which, for example, the shifting of lifting gas and resulting instability, was minimized.